The present invention relates to a method of manufacturing a molded plastic article and a glass mold for molding plastics.
More particularly, the present invention relates to a method of manufacturing a column or panel shaped molded plastic article having a desired molding shape on at least the circumference face of said molded plastic article comprising; manufacturing a column or panel shaped mother mold by using a material having high rigidity and heat resistance, the circumference face of said mother mold has a shape corresponding to the molded shape of the circumference face of said molded plastic article, manufacturing a cylindrical or ring shaped glass mold by the glass molding method, the inside face of which has a shape transferred from said shape of the circumference face of said mother mold, and molding a molded plastic article, circumference face of which as a shape transferred from said shape of the inside face of said resulting glass mold.
Hitherto a mold used for such as the injection molding or the compression molding of plastics and the like or press molding or glass molding is generally made by such as machining, an abrasive finishing of metal and the like.
To manufacture said mold made by manufacturing of metal as above described, a long time and a high expense are necessary. Accordingly, when said mold is used for the production in small amount such as for the trial production of parts, a unit price of parts may become high and it may be difficult to quickly supply said parts to users.
To manufacture said mold in a low expense and in a short time, a method comprising manufacturing a mother model by using a silicone rubber, forming a gypsum casting mold by using said mother mold and casting an alloy having a low melting point in said gypsum casting mold to manufacture a mold has been provided (TOKKO Hei 3-39776, TOKKAI Hei 5-301255, TOKKAI Hei 6-134815). Further, a means to solve the problem of contraction of said mold in said method has been provided (TOKKAI Hei 7-125014).
The facial accuracy of the polygon mirror as an optical element may include the plane accuracy, the inclination of plane from a base plane, the inclination error between faces adjoining respectively, the angle between planes adjoining respectively, the effective range and the like and it is difficult to mold said optical element in accuracy finer than 1 xcexcm by using said mold manufactured by above decribed method.
To manufacture a mold for parts having high accuracy, it may be necessary to combine a plural number of mold parts since it is difficult to form a cavity having high accuracy in one body excepting the mold for parts having a simple shape. For instance, to manufacture a mold for an optical element having high accuracy, a method of preparing a plural number of core mold parts for each optical face and combining said core mold parts have been provided. To set each core mold part to a fixed possition, guide ditches are formed in said mold in said method.
In said method in which a plural number of said core mold parts are combined together, processing common difference of each core mold part may accumulate such that each core mold part should have very high accuracy. Accordingly a lot of processing time is necessary, resulting in high cost.
The electrodischarge machining method or the electroforming method is a method to manufacture a mold in one body in which combining a plural number of core mold parts is not necessary.
By using said electrodischarge machining method, the accuracy of the resulting shape is rather high but since it is difficult to obtain the necessary surface roughness, the mirror polishing of faces to be optical face of said optical element may be necessary as finishing. Nevertheless, in the mold for manufacturing the optical element having a plural number of optical faces on the circumference face such as the rotating polygon mirror, since the inside face is the mold face which is a finishing face, it is difficult to finish the whole inside face in high accuracy, especially boundary part between the face and the face.
Further, said electroforming method has a probelm that the accuracy of the mold face becomes lower since the strain of electroformed plating layer is released when the master is removed. Although the amount of variation in this case is 1 to a few xcexcm, said amount of variation quantity may not be in a permitted range. Further, said electroforming method needs a longer time than abrasive finishing of metal.
Accordingly, an object of the present invention is to manufacture a column or panel shaped molded plastic article with high accuracy.
Another object of the present invention is to manufacture said molded plastic article at a low cost and in a short time.
Further object of the present invention is to provide a mold for a molded plastic article.
Briefly, said object of the present invention can be attained by a method of manufacturing a column or panel shaped molded plastic article having a desired molding shape on at least the circumference face of said molded plastic article comprising; manufacturing a column or panel shaped mother mold by using a material having high rigidity and heat resistance, the circumference face of said mother mold has a shape corresponding to the molded shape of the circumference face of said molded plastic article, manufacturing a cylindrical or ring shaped glass mold by the glass molding method, the inside face of which has a shape transferred from said shape of the circumference face of said mother mold, and molding a molded plastic article, circumference face of which has a shape transferred from said shape of the inside face of said resulting glass mold and a column or panel shaped glass mold used to mold plastics having a desired molding shape on at least its circumference face, said glass mold has a circumference face having a shape euqal to the shape of the circumference face of said molded plastic article and is manufactured by using a column or panel shaped mother mold made of a material having high rigidity and heat resistance and transferring the shape of the circumference face of said mother mold to the inside face of said glass mold.